Arrays of high-efficiency and high-brightness LEDs are at the moment considered for illumination. To make this feasible, individual LEDs have to be assembled at a fine pitch on a sub-mount.
At the moment thin-film processing on silicon substrates is already used to fabricate these sub-mounts. The use of silicon is advantageous, as silicon processing is well known, readily available and it is possible to produce metal structures on silicon at a sufficiently small pitch for use as circuitry for driving the LEDs. At the same time, silicon has a reasonable thermal conductivity to transport heat away from the LED.
Presently, the circuitry is grown on a thin isolating gate oxide to minimize the thermal resistance between the LEDs and the silicon substrate, and the LEDs are connected to the circuitry on top of the same.
In multi-LED applications, with several LEDs located at a fine pitch on the substrate, it is further desired to avoid cross-talk between LEDs, that is that light, which is emitted from the sidewalls of the LEDs, couples into a neighboring LED (optionally of a different color) and is absorbed there.
One way of preventing cross-talk between LEDs is to locate the LEDs in recesses in the substrate so that the walls of the recesses prevent the cross-talk between LEDs. However, the process of arranging the circuit pattern in such recesses is not straight forward, and thus, it has been suggested to lithographically build walls around the individual LEDs to prevent cross talk.
Another problem in multi-LED applications is that LEDs, and especially high power LEDs, dissipate a lot of heat energy when emitting light. This heat dissipation presents limits on how long or with which power the LEDs can operate. Thus it is much desired to obtain a good heat transport away from the LED. Conventionally this is done by placing the silicon-based sub-mount on a heat sink, with the side of the sub-mount housing the LEDs facing away from the heat sink. However, it would be advantageous to reduce the thermal resistance between the heat sink and the LED, as this would reduce the limitations due to the heat dissipation.
An alternative way of preventing cross-talk is disclosed in JP 61061458 A, to OKI Electric Ind Co Ltd. In this publication, a pattern of LEDs of mesa structure is arranged on a substrate. A photosensitive resin is then spin-coated on the substrate, thus filling the areas between the LEDs with a material forming light shielding regions between the LEDs. The circuit pattern is then arranged on top of the structure.
This method does however not provide improved heat transport away from the LEDs.
Thus it still remains a need for a LED-based lighting device that is easily produced and provides improved heat transport away from the LEDs. Especially there is a need for such devices with a reduced cross-talk between neighboring LEDs.